SMALL CELL AND INTERMEDIATE CELL FORMATION IN SPECIES OF DINOPHYSIS (DINOPHYCEAE, DINOPHYSIALES)

Abstract

Observations of two distinct size classes with similar shape in natural populations of Dinophysis Ehrenberg were first reported by Jorgensen in 1923 and intermediate forms exhibiting a continuum between the typical vegetative cell and a putative small cell by Wood in 1954. Focused attention on Dinophysis spp. associated with diarrhetic shellfish intoxications in the last decade has provided new examples of small cells in the genus, sometimes with contours dissimilar from the corresponding vegetative cells; dimorphic individuals; and large/small cell couplets. This work was based on in situ observations during intensive sampling for cell cycle studies of Dinophysis acuminata Claparéde et Lachmann, Dinophysis acuta Ehrenberg, Dinophysis caudata Saville‐Kent, and Dinophysis tripos Gourret; on laboratory incubations of D. acuminata; and on a thorough search of documented information on morphological variability of Dinophysis spp. During in situ division, most dividing cells exhibit a normal longitudinal fission, but some (1%–10%) undergo a “depauperating” fission, leading to pairs of dimorphic cells with dissimilar moieties. After separation and sulcal list regeneration, these dimorphic cells become D. skagii Paulsen, D. dens Pavillard, D. diegensis Kofoid, and D. diegensis Kofoid var. curvata‐like individuals, which can also be observed forming couplets D. acuminata/D. skagii, D. acuta/D. dens, and D. caudata/D. diegensis attached by their ventral margins. Small cells can grow again to large size, as shown in laboratory incubations of D. acuminata, thus partly explaining observations of thecal intercalary bands, and intermediate forms. The sexual nature of the small cells will not be unequivocally demonstrated until controlled germination of the alleged cyst forms is achieved, and some intermediate forms may correspond to undescribed stages after cyst germination. These observations suggest common patterns in the life cycle of Dinophysis spp. Intraspecific morphological variability of Dinophysis spp. in a given geographic area can largely be attributed to small cell formation, as a response to changing environmental conditions, and may be a part of the sexual cycle of these species. Small cells seem to be able to enlarge, leading to intermediate cell and further vegetative cell formation as part of a three‐looped life history pattern in Dinophysis.